This invention relates to a transparent substrate, in particular to a coated transparent sheet capable of withstanding heat treatment of a tempering or bending nature without degradation of the coating and adapted for example to be incorporated in a multiple glazing or a laminated glazing.
Many of the terms used to describe the properties of a coated substrate have precise meanings defined in relevant standards. The terms used in this description include the following, most of which are defined by the “Commission Internationale de l'Eclairage” (CIE).
In the present description, two standard illuminants are used: Illuminant C and Illuminant A, as defined by the CIE. Illuminant C represents average daytime light at a color temperature of 6700K. Illuminant A represents the radiation of a Planck radiator at a temperature of about 2856K. This Illuminant represents light emitted by car headlights and is particularly used in evaluating optical properties of vehicle glazings.
The term “luminous transmission” (LTA) as used herein is as defined by the CIE, that is the luminous flux transmitted through a substrate as a percentage of the incident luminous flux for Illuminant A.
The term “energetic transmission” (ET) as used herein is as defined by the CIE, that is the total energy directly transmitted through the substrate without a change in wavelength. It excludes the energy absorbed by the substrate (EA).
The term “color purity” (P) used herein refers to the excitation purity measured with Illuminant C as defined in the Vocabulaire International de l'Eclairage of the CIE, 1987, page 87 and 89. The purity is defined according to a linear scale in which a defined source of white light has a purity of zero and a pure color has a purity of 100%. For vehicle windows the purity of the substrate is measured from the external face of the window.
The term “dominant wavelength” (λd) used herein designates the wavelength of the peak in a range of wavelengths which are transmitted or reflected by the coated substrate.
The term “non-absorbent material” as used herein designates a material having a refractive index [n(λ)] which is greater than its extinction coefficient [k(λ)] over the whole of the visible spectrum (280 to 780 nm).
The term “emissivity” as used herein designates the normal emissivity of a substrate as defined in the Vocabulaire International de l'Eclairage of the CIE.
The term “haze” as used herein designates the percentage of diffused light transmitted by a material measured according to the ASTM D 1003 standard.
The Hunter coordinates L, a, b used herein measure the coloration of a material as perceived by an observer. They are defined and measured according to the ASTM D 2244 standard.
It has become more and more usual to apply a number of coating layers forming a coating stack to glass sheets to modify their transmission and reflection properties. Previous proposals for metal coating layers and dielectric coating layers in numerous different combinations have been made to confer chosen optical and energetic properties on glass.
Automotive glazings, in particular, are taking increasingly complex forms which require the glass of which they are made to withstand a bending heat treatment operation. In the architectural field it is also increasingly desired for glazings to have curved forms or for the sheets of glass from which they are made to have undergone thermal tempering for shock resistance and thus safety. However, the majority of coatings intended to be deposited on sheets of glass, particularly those deposited under vacuum, are not able to resist such heat treatment in a satisfactory manner. In particular, their optical properties are significantly degraded during such processes. Thus, it is necessary to apply the coating layers to the sheets of glass after the sheets of glass have taken their final shape or after they have undergone heat treatment which necessitates, particularly for curved glass, particularly complex deposition equipment. Such equipment must enable the deposition of uniform coatings on non-planar substrates.
It has been suggested to overcome this disadvantage by using coating stacks which incorporate coating layers comprised of materials which, when the substrate is raised to the temperature necessary for a tempering or bending heat treatment, can prevent the degradation of the optical properties of the coating stack for the duration of the heat treatment.
This degradation may in particular be attributed to, on the one hand, diffusion of oxygen from the atmosphere or from the dielectric coating layers of the coating stack which leads to oxidation of the metallic layers of the coating stack, and on the other hand to diffusion of sodium from the glass substrate into the coating layers of the coating stack.
European Patent Application No. 761618 describes a method of sputter depositing coatings on a glass substrate according to which the functional metal coating or coatings are surrounded by protecting layers comprising materials adapted to fix the oxygen by oxidation, in particular niobium. According to this document, the absence of degradation of the metallic layers is also due to deposition of the silver layer in a reactive atmosphere comprising at least 10% oxygen.
European Patent Application No. 336257 describes a glass substrate coated 25 with a coating stack which can resist heat treatment and which comprises two metallic coating layers deposited alternatively with three zinc stannate based dielectric coating layers. The first metallic layer is surrounded by titanium protecting layers and the second metallic layer is overlaid with a protection layer which is also of titanium. This material protects the metallic coating layers during heat treatment by being oxidized itself by combination with the oxygen atoms diffused in the coating stack.
European Patent Application No. 303109 describes a glass substrate coated s with a coating stack comprising a silver coating layer surrounded by two coating layers of combination of nickel and chromium which are themselves surrounded by two coating layers of a particular metal oxide. This product is intended to undergo bending by heat treatment in an oxidizing atmosphere during which its luminous transmittance increases significantly.
U.S. Pat. No. 5,584,902 describes a method of sputter depositing a coating stack capable of withstanding a bending or tempering type of heat treatment on to a glass substrate and which comprises a silver coating layer surrounded by two coating layers of a combination of nickel and chromium which are themselves surrounded by two coating layers of a silicon nitride.
Coating stacks such as suggested by these documents comprise protecting coating layers for the functional coating layers which before a bending or tempering type of heat treatment consist of non-oxidized metal. These protecting coating layers will be oxidized during heat treatment such that the optical properties of the coated substrate will be significantly modified during this process. In addition, it is necessary that these protecting coating layers are not oxidized to their interface with the functional metal layers so that the functional metallic layers are not subjected to oxidation. This is unfavorable for obtaining a high luminous transmission of the finished product.